The present invention concerns a process for removing chromium from chromium containing liquids. In particular, it concerns removing chromium from aqueous alkali metal chlorate solutions. More particularly, it concerns the removal of alkali metal dichromates from aqueous solutions of alkali metal chlorates by reacting said solutions with inorganic sulfur containing compounds. Most particularly, the present invention is concerned with the removal of sodium dichromate from sodium chlorate solutions.
Chlorate is an oxidizing agent used for the generation of chlorine dioxide, a bleach used in the manufacture of pulp and paper. Chromium, for example in the hexavalent form of Na.sub.2 Cr.sub.2 O.sub.7.2H.sub.2 O, is utilized as a catalyst in chlorate manufacturing processes. This catalyst promotes chemical reaction efficiency and inhibits explosive mixtures of hydrogen and oxygen from forming in the chlorate cells.
Historically, chromium was fed with the sodium chlorate/sodium chloride solutions into chlorine dioxide generators. However, the industry is converting over to new, higher capacity methanol driven generators which do not perform as well when chromium is present in the chlorate feed. There is also an environmental concern. Chromium in the hexavalent form is known to be a noted carcinogen. Chromium which is fed to a chlorine dioxide generator eventually ends up in the pulp mill waste treatment system in the trivalent form which in the past has been acceptable to the Environmental Protection Agency. However, the Environmental Protection Agency now requires that total chromium be reported from any and all discharges from pulp and paper mills or from any other user of chromium containing products. Therefore, the chromium now needs to be contained in any process which produces chromium or uses chromium.
Kaczur et al., U.S. Pat. No. 4,259,297, discloses a multistep process for removing hexavalent chromium from impure aqueous alkali metal chlorate solutions. The pH of the impure aqueous solution is adjusted in a first step to a first intermediate pH in the range from about 9 to about 13 before mixing with the reducing chemical. An inorganic sulfur compound is then added. Then, in a second intermediate pH adjustment step where the pH is adjusted from about 2 to about 4, the inorganic sulfur compound reduces the chromium to trivalent and divalent chromium. Finally, in a third intermediate pH adjustment step, the pH is adjusted from about 6 to about 8 and the reduced chromium is precipitated as hydroxides.
Japanese Patent 119493 (1980) discloses a multistep method of removing low levels of chromium (i.e., 0.5 to 50 ppm) from reconstituted chlorate solutions from crystallizers. In this multistep process, hexavalent chromium is reduced by adding sulfite at a pH of from 3 to 6. The amount of sulfite added must satisfy a complex equation which is a function of pH and concentrations. The patent then states that the pH is adjusted from 9 to 11 to precipitate the chromium as hydroxide, but no details are provided as to how this can be accomplished.
Both of the above-described references produce chromium hydroxides which are well known to be difficult to filter. Kaczur discloses an expensive and awkward multistep solid-liquid separation process employing two centrifuging stages and two filtration stages. Japanese Patent 119493 does not address the solid-liquid separation problem. No commercial process exists employing either of these two processes.
In the present invention, there has been discovered a set of conditions which allow the chromium to be simultaneously reduced and precipitated to an easily filtered metal oxide in a single stage process. Because of the process conditions of the present invention, solid-liquid separation can be accomplished on a full scale basis in a single filtration step without the use of clarifiers or centrifuges. A usable chromium precipitate is produced which can be added directly to a chlorate cell as a direct replacement for hexavalent chromium without further processing, essentially creating a closed loop where the benefits of the chromium to the process are retained indefinitely and the chromium never leaves the process.
The present invention offers many advantages: (1) There is no release of noxious by-products such as sulfur or chlorine based gases which are characteristic of most other chromium precipitation processes. (2) Carrying out the process at close to neutral pH eliminates the possibility of Cl.sub.2 or ClO.sub.2 generation by the break down of chlorate under acidic conditions, and solutions are safe for storage at all points in the process. (3) The sulfur based reducing agents are readily available and, when used in the alkaline solution form, are safe and easy to store and handle without the release of SO.sub.2. (4) There are no by-product reaction contaminants in the chromium free chlorate product which could adversely affect the operation of the chlorine dioxide generators. The reaction product is sodium sulfate which is normally present in the salt cake produced by the generators. (5) The resulting chromium precipitate, which is mainly chromium oxide, is more easily filtered than the gelatinous chromium hydroxide precipitates produced by the prior art processes, and filtering can be done inexpensively in a single stage with a filter without the use of a centrifuge. The chromium oxide precipitate collects on the filter as a dense filter cake, therefore the amount of liquid product recycled back to the process is minimized. In applications where the chromium needs to be disposed of or taken to another site for reprocessing (e.g., removing chromium from waste water), having the chromium sludge in the form of a dense filter cake minimizes the total volume of hazardous chemical which has to be handled. (6) The precipitate is in such a form as to be easily converted into a usable form, unlike prior art processes which do not yield chromium in a usable form. (7) The filtered precipitate is in such a form that it is immediately reoxidized back to hexavalent sodium bichromate when mixed with hypo containing solutions or when added to an electrochemical cell where hypochlorite is present. In the chlorate process, the filter precipitate can be added directly back to the chlorate cells as a replacement for the sodium bichromate. (8) There are no flocculating agents or by-product reaction contaminants such as elemental sulfur in the precipitated chromium sludge which would require further processing before the sludge is reused. (9) The present process will work with all concentrations of chlorate and chloride by making slight process temperature adjustments. The process will also work for removing hexavalent and trivalent chromium from waste water streams. (10) The chromium removal process of the present invention, coupled with an upstream Huron high temperature continuous dehypochlorination reactor, can remove essentially 100% of the hypochlorite present in the chlorate cell liquor and eliminate the pH control step normally associated with the dehypochlorination reactor. (11) Unlike prior art processes which utilize several pH adjustment steps from extremely alkaline to extremely acidic for the reduction and precipitation of chromium, the present invention involves a pH close to neutral which is optionally followed by one pH adjustment to the alkaline range. (12) Reaction temperatures are relatively low so exotic materials of construction are not needed.
The present invention allows essentially 100% of the chromium oxide from a chlorate liquid product to be safely and inexpensively removed and recycled without the generation of by-product reaction contaminants and without the release of noxious by-product sulfur or chlorine based gases characteristic of prior art processes. The chromium oxide precipitate can be added back to the chlorate process as a catalyst replacement without further processing. In addition, the process can be utilized to remove chromium from electroplating solutions and chromium plant effluents as well as removal and recovery of chromium from waste site leachants.